Which one I’d prefer... That’s like picking my favorite child. Both are excellent, really, and it’d come down to personal preferences as to what to recommend to a given person. The U12t is slightly thick and slightly downsloping save the pick up in the upper treble, a more agreeable tonality in general, but perhaps too dark or suppressed in the upper midrange for some. The Viento is bright and lean with tons of upper mids and mid-treble, enough to turn off some people.
Regarding bass quality, with BA bass you’re always compromising something. With the U12t you get extension but trade off proper thick slam, texturing, layering, transients and timbre. With Viento you get something very close to DD bass transients and control, but you don’t get the extension nor do you get the timbre, texture or layering. Ultimately no BA bass gets all of these right - if you want all of those then DD is the way to go.
Technicalities wise I would say they’re very close, but give the nod to the U12t. It’s very good with microdetail stuff and dynamics, ahead of possibly every other full BA on the market. I’d need to revisit the VE8 to confirm this but as it stands I think the U12t does those things better than anything else I have heard in recent memory. The Viento’s dynamics, as I have noted, as just behind the U12t and its younger (or older?) brother the NT-6. Not by a massive margin, but a margin nonetheless.
On the other hand, the Viento’s texturing is way more upfront and aggressive than the U12t. Some people will prefer this. It’s more outwardly resolving of surface texture, especially in the upper mids. It also has a more coherent tonality than the U12t. Not to say that the U12t is incoherent, but the upper midrange on it sounds slightly veiled at times. I suspect this is thanks to the suppression past 2.2khz as well as the lack of mid treble, but in any case the Viento doesn’t have this or anything similar in any other frequency range. I hesitate to call it this, but it feels better tuned.
Also, don’t forget price. You can grab a Viento for a literal fraction of the U12t’s asking price, if you know where to look. Though admittedly knowing where to look is the difficult bit in this situation. In which case it comes down to availability and resourcefulness.
Hope this helps.